The present invention relates to a method for testing the raster scan geometry, such as the linearity of the scan, of a television display apparatus, such as a television monitor, or a television camera.
Modern applications of television display apparatus, such as closed circuit television monitors, demand much greater accuracy in the raster scan geometry, particularly in non-broadcast applications. For example, computer generated images on the screens of cathode ray tube devices are developed with very great timing accuracy, and systems are currently being offered in which the analysis of data can be made by taking measurements directly off the image displayed on the screen. For example, in computerized medical X-ray technology, it is desirable to make measurements of tumor size directly off the television display, so size and accuracy of the displayed image are of paramount importance. Scan size may be specified to within a few thousandths of an inch, and stability over the short and long term must assure that size and position will remain within a few thousandths of an inch through cold start, continuous operation, and varying ambient temperatures. In order to be able to test the more precise television devices that have been developed recently, new test procedures and apparatus that can conform to such precision have become necessary.
The generally accepted method of measuring linearity of a cathode ray tube display apparatus or television camera is the comparison of an electronically generated reference grid compared to a test chart, wherein the chart is either projected on the screen or viewed by a camera and the electronically generated images produced on the screen. The chart that has been in use for many years is the EIA linearity chart, which is sometimes referred to in the industry as the "Ball" chart. The chart comprises a plurality of rows and columns of black circles on a white background wherein the center of each circle is also white. The chart is standard in format and is matched to a 525 line raster having a scan rate of 525/60. Linearity charts for other scan rates not compatible with a 525/60 scan rate chart have not been developed.
In testing a television camera, the camera is arranged to view the linearity chart, and the image of the chart is then produced on the television monitor connected to the camera. An electronically generated pattern, such as a bar pattern or dot pattern, is superimposed on the camera output signal and the combination signal is displayed on the television monitor for analysis. The camera is manipulated so that the image of the chart is matched as closely as possible to the electronically generated bar or dot pattern, that is, as many as possible of the dots or intersections of the vertical and horizontal bars are caused to coincide with the centers of the circles of the graphic elements making up the linearity chart. The amount of non-linearity can be measured by noting the positions of the dots or bar intersections relative to the graphic elements, in the case of the linearity chart individual circles surrounding a center space.
In measuring the linearity of a display device, such as a television monitor, the linearity chart would be projected onto the screen of the monitor by a slide projector, although the chart could be directly "projected" by using a transparent overlay affixed to the face of the screen. The electronically generated dot or bar pattern produced by a television test pattern generator would be produced on the screen, and by the monitor adjusted so that as many as possible of the dots or bar intersections would fall within the centers of the projected chart circles. The electronically generated pattern, if accurately produced, will match the chart when the scan is linear and without other distortions, and a perfectly linear scan is indicated if each electronically generated dot or bar intersection falls within the exact center of the chart circles.
A fundamental problem with the current testing procedure is that the traditional linearity chart is configured to match the bar or dot pattern from a traditional broadcast studio synchronizing generator, which produces an average horizontal bar spacing of 17.5 lines in the 525 line raster. In the standard pattern used with the linearity chart, the vertical bars are made up of 20 elements per total horizontal line, including blanking which is 17.5% of the total line. Thus, there are 17 visible vertical bars centered in the active area, and the bars at the left and right are set in from the edge of the display by 1.25% of the total horizontal line time. The pattern comprises 15 equally spaced horizontal bars, one of which is invisible due to the 7.5% vertical blanking interval. This results in 14 horizontal bars being visible with the bars at the top and bottom of the raster being set in from the respective edges by 2.85% of the total vertical field time.
Although the dot or bar pattern described above is compatible with the standard linearity chart, many scan rates are in common use, most of which cannot match the standard linearity chart due to the fundamental raster structure. Calibration of the horizontal bars in the vertical plane of the pattern based on 15 elements spaced equally in the total vertical television field, including blanking time, can be easily accomplished because the total number of horizontal lines (525) is equally divisible by the 15 horizontal bar elements. The vertical field is produced at 60 hertz for all scan rates described by present U.S. standards; therefore, the repetition rate for horizontal bar elements is 900 hertz, requiring 1,111 microseconds between the elements. It is necessary, in an accurately generated pattern, for the horizontal bar element to occur for one complete scan line, positively locked so as to preclude drift or "run through" that will permit intensification of partial lines. Otherwise, an error of at least one scan line width can occur in the display of the pattern.
These factors present a conflict because 1,111 microseconds is not an even multiple of the scanning line for any of the standard scan rates. The 525/60, 675/60 and 945/60 scan rates can be accommodated by an alternating count technique, but for other rates, a precise and straightforward measurement is possible only by the use of linearity charts that are modified or customized to match the raster structure of each rate.
Certain scan rates whose number of scan lines in two fields (one frame) is divisible by 15 are usable with the standard linearity chart by using a horizontal alternate count. For example, a scan rate of 675/60 comprises 675 horizontal lines and is divisible by 15, the number of horizontal bars or rows of dots to be produced according to generally accepted standards. For other, non-equally divisible, scan rates, the standard linearity chart has been used but it was necessary to vary the chart-pattern matching procedure in order to artificially constrain the pattern or size of the chart to match. For example, the chart projected image could be reduced in horizontal height in order to match up with the geometric centers of the horizontal rows of dots or horizontal bars, but this would also result in a narrowing of the vertical bars, and it would then be necessary to modify the controls of the monitor to narrow the vertical bar spacing. Furthermore, a reduction in the size of the projected image of the chart results in a reduction in the size of the circles, so that the radii of the circles no longer accurately represent a percentage of the height of the display, which is the basis for quantitative evaluation. Electronically changing the location of the vertical bars or vertical columns of dots in the generated pattern results in a distortion of the pattern and automatically throws an error into the image displayed on the screen. In other words, the pattern displayed by the television monitor is not displayed under normal operating conditions, but the controls of the monitor are manipulated to artificially adjust the positions of the vertical columns. When the test procedure is completed and the monitor is readjusted to its normal operating setting, then there is no assurance that the same degree of linearity is present after the test as during the test because the control parameters are not the same. This reduces the precision of the linearity test procedure and one can never be certain of the quantitative amount of non-linearity under normal operating conditions.